@acebaur correct. For frightful touch attacks Any success other than a natural 6 you roll for wounds and saves. You roll all attacks for a unit for a single weapon at once. eg 4 attacks, roll all 4 to hit. (See Multiple Attacks p.232 Core Book) Mortal wounds are special, and don’t need wound rolls and (unless your opponent has a special save) doesn’t get a save roll against it.

For Any Of the dice that roll a 6 put those aside. They cause MORTAL WOUNDS and no hit or save is required.